1. Field of the Invention
The invention relates to orthogonal frequency division multiplex (OFDM) communication systems and more particularly to synchronization and channel estimation for an OFDM communication system.
2. Description of the Prior Art
Several synchronization and channel parameters must be estimated before symbol decisions can be made in systems using coherent multicarrier communication such as orthogonal frequency division multiplex (OFDM) systems. A receiver must identify the start of a packet or frame (time synchronization), adjust for offsets in sampling phase and carrier frequency (frequency synchronization), and equalize for the channel impulse response (channel equalization). Inaccurate synchronization leads to inter-symbol interference (ISI) or inter-carrier interference (ICI), both of which degrade the overall bit error rate (BER) performance of the system. Errors in channel estimation also lead to BER degradation.
Most recent multicarrier system standards require guard band symbols of zero level and also symbols known as pilots. The guard band symbols are used to help contain the spectrum of the signal within the spectrum that is allowed for the system. The system pilot symbols are interspersed with user data symbols. FIG. 1A is a chart showing amplitudes for guard band zeros, user data symbols and system pilot symbols on a vertical axis versus a symbol index from −N/2 to +N/2 on a horizontal axis where N is the total number of symbols in a symbol block. The amplitudes of the user data symbols go up and down as they are modulated with information from a user. The system pilot symbols have known modulations that may or may have constant amplitude.
Conventional OFDM systems use frequency domain pilot-assisted channel estimation in order to measure the channel attenuations on those carriers for channel equalization. Unfortunately, the conventional frequency domain pilot-assisted channel estimation methods require an additional frequency domain filter for interpolating the channel response between the carriers of the pilots and the BER performance of such systems is sub-optimal depending on the choice of this interpolation filter.
Several existing OFDM systems use special time domain structures for time synchronization. For example, IEEE 802.11 describes time synchronization using a preamble and digital audio broadcasting in Europe uses null symbols. However, such special synchronization structures reduce channel efficiency and in any case are not available in some OFDM standards.
Recent multicarrier standards such as Digital Video Broadcasting (DVB) and OFDM access (OFDMA) mode in IEEE 802.16 have eliminated special time domain structures and rely instead on a part of the OFDM packet called a cyclic prefix (CP) for synchronization. This method has the advantage of greater efficiency because the cyclic prefix always exists in OFDM signal packets as a guard to eliminate ISI between successive packets.
FIG. 1B is a block diagram showing the cyclic prefix (CP) method of the prior art for synchronization. In the CP method, an OFDM block includes OFDM samples for the cyclic prefix that are prepended to the beginning of the OFDM block. The prepended cyclic prefix OFDM samples are duplicates of a predetermined number of OFDM samples from the end of the OFDM block. The OFDM samples separated by the total number of samples in the OFDM block minus the number of cycle prefix samples are complex multiplied. The resulting products are passed to a shift register. Then, the registers are summed to provide a correlation function. Peaks in the correlation function provide information for time and frequency synchronization.
Unfortunately, a receiver using the cyclic prefix method for time synchronization of a signal received through a dispersive channel is prone to intersymbol interference (ISI) that causes degraded sensitivity and an irreducible error floor. In order to avoid both the inefficiency of the special time domain structures and the ISI that results from the CP method, workers have proposed frequency domain pilot-assisted time synchronization methods using phase rotation observed on the OFDM pilots.
The frequency domain pilot-assisted time synchronization methods have the advantage that the pilots are required by the existing OFDM standards for channel equalization. However, existing frequency domain methods require additional receiver hardware for computing the phase rotations on the pilot tones, and the measurements are artificially decoupled from the effect of the channel itself on the various tones. Thus, these methods lock to the center of gravity of the channel impulse response as opposed to its dominant path, leading to ambiguity in where to start the demodulation window, and causing an associated loss in received signal energy. The resulting synchronization performance is often sub-optimal in a dispersive channel.
Frequency synchronization using the cyclic prefix is also known for existing OFDM systems. Unfortunately, the range of frequency offset that is determined with the cyclic prefix is limited to ±½ the subcarrier spacing. Moreover, the frequency synchronization cyclic prefix method also suffers from ISI. In order to avoid these limitations workers have proposed frequency domain pilot-assisted frequency synchronization methods using the pilots in the OFDM standards. However, because the effect of a carrier offset is energy leakage between adjacent carriers (ICI), a frequency offset is very difficult to estimate in frequency domain. Methods exist to reduce the ICI by detecting the collapse of the orthogonality condition using different windowing and filtering techniques. However, these methods have not been robust up to the present time.
There continues to be a need for improvements in the signal processing apparatus and methods for achieving time and frequency synchronization and channel equalization in multicarrier communication systems.